The invention relates to a direct fuel injection method of the type using a pre and a main fuel injection for a diesel engine.
U.S. Patent specification 2,356,627 describes such a method in which a preset quantity of fuel is introduced, subdivided into a pre and a main injection, into the combustion space of the internal combustion engine. In detail, the beginning of the preinjection occurs in this process approximately 14.degree. before top dead center of ignition and lasts approximately until 10.degree. before top dead center. After a pause of approximately 9.degree.-10.degree. crank angle, the main injection finally begins approximately before top dead center. On the ratio between pre and main injection quantity, the above document only carries the information that the preinjection quantity can be both smaller than, equal to or also greater than the main injection quantity. In the method according to U. S. Patent specification 2,356,627, it is also provided to select the preinjection quantity and the interval between the end of preinjection and the beginning of the main injection in such a manner that the main injection jet still finds in the combustion space an open flame from the preinjection quantity which is just being converted. Although this type of fuel injection reduces the pressure gradient during the combustion of the main injection quantity, and thus also the combustion noise, cracking of individual fuel molecules introduced with the main injection quantity can very easily occur as a result of the open flame in the combustion space.
This results in increased particulate emission so that a strictly predetermined blackening number is already reached at a relatively low effective mean pressure which lastly encumbers the performance of the internal combustion engine.
Retaining the above referenced angles for pre and main injection, these said disadvantages increasingly occur in higher speed ranges because the time available for forming the mixture decreases with increasing speed so that the combustion, particularly of the main injection quantity, is displaced extremely far into the expansion cycle which leads to considerable losses in efficiency and thus to increased specific fuel consumption.
The invention is therefore based on the objects of creating a fuel injection method by means of which a short ignition delay and an increase in mixture forming energy can be achieved while mantaining the lowest possible specific fuel consumption.
According to the invention, these objects are achieved in accordance with a first preferred embodiment of the invention wherein direct fuel injection is carried out in the following manner:
At engine idling speeds, preinjection is initiated within the range of 10.degree.-16.degree. crank angle before piston top dead center position, main injection is initiated after 2.degree. crank angle after piston top dead center, the interval (.alpha.p) between the end of preinjection and the beginning of main injection is between 3.degree. and 14.degree. crank angle, and the preinjection quantity is between 10% and 20% of the total preinjection and main injection quantity of idling load.
At engine rotational speed at maximum power and full load, preinjection is initiated within the range 20.degree.-30.degree. crank angle before piston top dead center position, main injection is initiated after 15.degree. crank angle before top dead center, the interval (.alpha.p) between the end of preinjection and the beginning of main injection is between 3.degree. and 26.degree. crank angle, and the preinjection quantity is between 1% and 5% of the total preinjection and main injection quantity.
In certain preferred embodiments, the preinjection at initial idling speeds is dispensed with and fuel is supplied only by the main injection. The idling speed preinjection described above is then initiated at engine speeds shortly above the initial idling speed.
In certain preferred embodiments, the preinjection and main injection timing vis-a-vis the piston crank angle is continuously adjusted as a function of engine speed within the above-noted limits for idling and rotational speed at maximum power and as a function of engine load within the above-noted limits for idling and full load.
Due to the fact that the preinjection quantity is very small and reaches the combustion space only relatively late, only extremely slight additional energy needs to be expended by the piston as a result of which an advantageous specific fuel consumption of the internal combustion engine is obtained. A further advantage of a relatively later injection of the very small preinjection quantity results from the fact that this fuel reaches the combustion space at a time at which a high compression pressure is already present. The preinjection quantity therefore ignites early and completely burns within a short time interval due to its small quantity so that the main injection can take place at only a slight interval after the end of the preinjection. In addition, it is advantageous that, on the one hand, a relatively high temperature level occurs in the combustion space before the main injection and, on the other hand, the gases located in the combustion space have an increased flow velocity. Thus, due to the relatively high temperature level, rapid evaporation of the injected fuel and, due to the increased flow velocity, good homogenization of the cylinder content is achieved after the beginning of the main injection. Rapid evaporation of the fuel and good formation of the mixture require a short ignition delay by means of which the rate of heat release is lowered at the beginning of the combustion of the main injection as a result of which less heat can also flow off into the cooling water. The burning out of the fuel is accelerated by reaction products from the preinjection so that the combustion process occurs at a relatively constant rate of heat release. Thus, the same advantages as in a prechamber engine are achieved with respect to noise and waste gas emissions; nevertheless, the consumption specific advantages of the direct injection method are retained.
In addition, the small ignition delay also leads to the pressure in the combustion space not being able to rise to values which are too high as a result of which the peak temperature in the combustion space and thus the formation of nitrogen oxide is clearly lower compared with conventional direct injection methods. Finally, another advantage of the injection of as low a preinjection quantity as possible arises from the fact that a relatively large supply of fresh air is still available in the combustion space at the time of the beginning of the main injection so that no significant increase in the formation of particulate matter occurs.
An effect which is also advantageous for particulate emission is that, with the method according to the invention, complete conversion of the preinjection quantity is always ensured, starting from the lower speed range up into the range of rotational speed at maximum power before the beginning of the main injection. Referred to a strictly predetermined blackening number, an increased effective mean pressure and, at the same time, naturally also an increased internal combustion engine performance can thus be achieved.
A speed-dependent displacement of the beginning of preinjection and of the beginning of main injection in the manner according to the invention has the advantage that the conversion of the main injection quantity always occurs at the optimum time with respect to the efficiency of the internal combustion engine.
Since a diesel engine is always run with a large amount of excess air in the idling range and thus the emission of pollutants is relatively low, in any case, it is also possible to completely dispense with the preinjection in this operating range and to introduce the entire quantity of fuel into the combustion space with the main injection which begins approximately 2.degree. after top dead center.
Another advantage of the direct injection method according to the invention arises from the fact that the compression ratio of the internal combustion engine can be reduced in the direction of a value which is optimum with respect to thermal and mechanical efficiency without having to fear an increased ignition delay and the associated disadvantages.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.